Isotropic microwave reflector



March 21, 1967 J. B. B|AuER 3,310,804

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March 21, 1967 J, BRAUER 3,310,804

ISOTROPIC MICROWAVE REFLECTOR Original Filed June 18, 1963 4Sheets-Sheet 4 INVENIOR. c/flJ'EP/f 5.316405? United States Patent3,310,804 ISOTROPIC MICROWAVE REFLECTOR Joseph B. Brauer, Rome, N.Y.,assignor t0 the United States of America as represented by the Secretaryof the Air Force Original application June 18, 1963, Ser. No. 288,840.Divided and this application Dec. 9, 1965, Ser. No. 516,207

2 Claims. (Cl. 34318) The invention described herein may be manufacturedand used by or for the United States Government for governmentalpurposes without payment to me of any royalty thereon.

This application is a division of my copending application Serial No.288,840, filed June 18, 1963.

This invention relates to microwave reflectors, and more particularly tothe fabrication of isotropic microwave refiectors.

An isotropic microwave reflector is a reflector that reflects the waveback in the same direction as the incident wave regardless of thedirection of the incident wave. This will occur by using cornerreflectors which is the name commonly given to devices constructed withthree mutually perpendicular reflecting planes whose intersection lie ata common point about an axis about which the planes are equispaced.Incident electromagnetic energy entering the open face of the invertedpyramid formed by the planes is reflected from two planes of thisreflector in such a manner that it is returned parallel to the incidentpath with no reduction in total incident energy save that due to qualityof the reflective surface.

The invention is based on the use of numerous individual cornerreflectors mounted with their open faces lying on the plane surfaces ofa regular polyhedron. Since the open face of the corner reflector formsan equilateral triangle on a plane normal to the axis of the reflector,there is the further requirement that the appropriate regular polyhedronhave equilateral triangles for external faces or other polygon surfaceswhich can be extended in such a manner to form triangular faces.

The ideal isotropic reflector should therefore consist of a regularpolyhedron with equilateral triangular faces and should approximate aspherical body, presenting a uniform array of individual cornerreflector to radiation incident from any direction. The relatively broadbeam or angle of incidence tolerance (deviation from the true axis) forradiation which the corner reflector will accept and return allows theuse of polyhedra with relatively few faces, only roughly approximating asphere.

An object of this invention is to provide for a radiant energy reflectorthat will cause a reflected wave to be parallel to the incident wave.

Another object is to provide for a method of calibrating radar.

Another object is to provide for a method of making aircraft or othervehicles easily detected by radar.

Another object is to provide for a better camouflage in order to confuseenemy radar.

These and other objects and features of the inevntion will more fullyappear from the drawings in which:

FIG. 1 shows a single corner reflector;

FIG. 2 shows a front view of a solid sector of the polyhedron showing asingle equilateral triangular face further divided into four smallerequilateral triangular faces;

FIG. 3 shows an isotropic microwave reflector in the form of anoctahedron;

FIG. 4 shows a solid sector of the polyhedron in form of a pentagonwhich has been further divided into five equilateral triangular faces.

FIG. 5 shows an isotropic microwave reflector in the 'form of polyhedronof multiple sides sufiiciently numer- 3,3103% Patented Mar. 21, 1967 iceoils to approach a spherical configuration; the figure also showing amethod of anchorage to ground;

FIG. 6 shows the mold used to form a solid sector of the polyhedronshown in cross section along lines at 66 of FIG. 2;

FIG. 7 shows a mold designed to cast an isotropic microwave reflector inone piece;

FIG. 8 shows a method of calibrating a ground mounted radar using aground mounted target and a method of serial calibration using a targetsuspended from a balloon;

FIGS. 9(a) and 9(b) show methods of mounting isotropic microwave targetsin an aircraft for better radar detection;

FIGS. 10(a) and 10(b) show methods of rotating the polyhedron to preventeasy radar detection; and

FIG. 11 shows a method of using the microwave reflector for purposes ofcamouflage.

Referring now to the drawings in more detail, FIG. 1 shows the basiccorner reflector made up of three mutually perpendicular right triangles11, 12, 13 joining at vertex 14 and forming equilateral triangle 15.

As shown in FIG. 2 each equilateral triangle 16 can be divided into foursmaller equilateral triangles 17, 18, 19, and 28 each in turn having thesame properties as shown in FIG. 1. This process of dividing theequilateral triangle can be continued in order to make the polyhedronmore closely approach a sphere thus reducing or eliminating slightvariations in returned radiation as .a function of position of theincident beam.

The simplest form of a polyhedron suitable for use as an isotropicmicrowave reflector is the tetrahedron having four triangular faces.

FIG. 3 shows a variation of the polyhedron in the form of an octahedronwhich has eight corner reflectors and eight equilateral triangularfaces. Four corner reflectors 28, 29, 3t), 31 are shown. Construction ofthe reflector is shown with foam base 89 and reflecting surface 90.

Other variations of the polyhedron are possible such as the dodecahedronwhich has twelve pentagon faces and as shown in FIG. 4; each pentagonface can be further divided into five equilateral triangles 22, 23, 24,25, 26. Hence the dodecahedron would have sixty triangular faces.

FIG. 5 shows an isotropic microwave target 1313 showing small cornerreflectors 15 sufficiently numerous to cause the polyhedron to thedesired surface. Construction is shown with foam base 131 and reflectingsurface 132.

There are many possible variations of simple or mixed polyhedronwhich-can be used to achieve the desired size and distribution ofindividual corner reflectors composing the isotropic reflector. Thesevariations may be applied in order to match the measurements of theindividual reflector to the frequency radiation for which it is to beused by increasing the total number of individual reflectors in a singleface of the polyhedron or to make the polyhedron more closely approach asphere, thus reducing or eliminating slight variations in returnedradiation as a function of position of the incident beam.

Referring to FIG. 8, the isotropic microwave reflector can be used forcalibration of ground radar sets using a ground mounted target 57 todetermine microwave return and geographic location, thereby eliminatingprecise surveying of a reflector location and angular orientation. Theonly measurement needed would be distance from the radar set 58. To makeaerial calibration of ground mounted radar, the isotropic microwavereflector 55 could be suspended from balloon 56 sent up at a desireddistance from the radar set 53.

Radar .pulses 51 transmitted, from antenna 5-2 strike the ground mountedmicrowave reflector 57 or the suspended reflector 55 and are reflectedback to the antenna with little loss of energy.

The invention can also be used to increase radar crosssection ofaircraft for easier and more precise detection by friendly radar in airand ground traffic control systems, or to cause a small aircraft toappear much larger to an enemy, or to cause a single plane to appear asmore than one, generating confusion prior to or during attack. In FIG.9(a) small reflectors 61 and 62 can be mounted in the wing tips 63 and64 of aircraft 65, or in FIG. 9(b), reflector 66, can be mounted on fueltank 67 of air-craft 69.

The reflector could be mounted so as to rotate in order to eliminate thehigh reflection when desirable as shown in FIG. (a). A polyhedron 71, ismounted on the structure 72 of an aircraft in order to increasereflectivity. If it becomes necessary to eliminate this highreflectivity the polyhedron can be rotated 180 which would then expose anon-reflecting surface 73. This rotation can be accomplished by causingrod 75 to move which is connected to rack 102 which turns gear 77 whichin turn is connected to the upper end of axle 81 running throughpolyhedron 71. At the same time rod 78 is caused to move in the oppositedirection as rod 75 by linking rod 78 to rod 82 by means of lever 83turning at pivot point 84 which is attached to structure 72 by bracket85 and then causing rod 82 to move in the same direction as rod 75. Rod78 is connected to rack 79 which turns gear 80 which is connected to thelower end of axle 81. With rods 75 and 78 moving in opposite directions,the polyhedron is caused to rotate. Rods 75 and 82 are actuated byelectric current in coils 86 and 87 which encircle the rods.

Referring to FIG. 10(b) which shows the switch assembly that controlscurrent flow in coils 86 and 87, when button 93 mounted in panel 94 isdepressed, crossbar 95 joins contacts 96 and 97 thus closing the switch.Voltage source 98 causes electric current to flow in wire 99 which isconnected to coils 86 and 87 at the same time current is caused to flowin coil 100 which encircles rod 101 that is connected to button 93. Rod101 and coil 100 act as a solenoid thereby keeping the switch in theclosed position. Rod 101 enters chamber 104 which contains spring 105.When button 93 is depressed spring 105 is compressed between collar 106and detent 107.

In order to cause the polyhedron to reverse its rotation and againexpose the side of high reflectivity, switch 103 which is normallyclosed is opened thereby breaking the circuit. Current ceases to flow incoil 100 and spring 105 acting on collar 106 causes rod 101, cross bar95, and button 93 to be raised to its original position. Current alsoceases to flow in wire 99 and coils 86 and 87 at FIG. 10(a) and a springnot shown acting on axle 81 returns the polyhedron to its originalposition of high reflectivity.

The invention can be used as camouflaging device in order to deceiveenemy radar by using a complex of reflectors to duplicate reflectingcharacteristics of existing buildings or landmarks or totallyobliterating a target by saturating the target area with high returnreflectors as shown in FIG. 11. Aircraft 1 11 sends out radar signals112, 113, 114, and receives reflected signals 115, 1-16, 117. Althoughisotropic microwave reflectors 118 and 119 are smaller than structure120, they will appear to be of equal size to the radar in aircraft 111due to the high reflectivity of the isotropic microwave reflectors.

Another use is as an aid to investigation of various parameters ofmicrowave systems under development or test, such as field intensitydistribution.

The polyhedron can be fabricated either by casting solid sectors andthen assembling them or by casting the entire body of the reflector inone piece.

Where suflicient numbers of the reflectors are required to justifyhigher initial mold cast, a suitable mold, split to allow for removal ofthe cast part, could be designed to enclose the entire body of thereflector and the body could be cast in one piece of foam plastic orother light weight material. Such a mold is shown in FIG. 7 which issplit at line 3 to allow it to be open for removal of the polyhedron.The mold is composed of drag assembly 39 and cope assembly 40 which arekept in alignment by guides 41 and 4-2 which are press fitted into dragassembly 39. The mold material can be poured through feed line 86.

Where only a few reflectors of a particular size or configuration arerequired a mold such as shown in FIG. 6 could be designed to enclose thevolume contained in solid sector 16 of the polyhedron. The solid sectormay then be formed by casting a low density from plastic or othermaterial in this mold cavity. The material is poured into the feed line32 and then into the mold 33 which includes drag assembly 34 and copeassembly 35. Upper platen 37 is attached to shank 36 which is in turnattached to ram of press not shown to insure proper pressure on mold 33.

The conductive coating required to reflect microwave radiation can beapplied in either of two manners. A hot metal spray coating can beapplied to mold over a mold release coating and the foam plastic orother material would be subsequently introduced so as to bond the spraycoating during the casting operation. Another manner of coating wouldbe, after casting is completed, to spray, dip, or paint application of ametal paint, or other conductive coating which would adhere to the basematerial.

If desired, the entire reflector could be made of sheet metal as ahollow polyhedron or may be cast of a foamed metal such as aluminum ormagnesium for a more rugged structure requiring no separate reflectivecoating.

Weather proofing of the reflector may be accomplished as a final step inthe fabrication by coating with an epoxy or other weather-proof organicresin, or spraying on a thin layer of foam.

What I claim is:

1. An aircraft electromagnetic energy reflecting system comprising:

(a) first reflecting unit including a polyhedron having a reflectingsurface and a non-reflecting surface, said reflecting and non-reflectingsurfaces facing in opiPOSitC directions along a common axis of symmetrypassing through said surfaces, the reflecting surface having amultiplicity of sides sufficiently numerous to produce an overallreflecting configuration that is substantially hemispherical, each ofsaid sides including in the surface thereof a depressed portion forminga re-entrant angle, all of said depressed portions serving as cornerreflectors;

(b) and means including a rotatable axle aflixed to said polyhedron forrotating the polyhedron through an angle of degrees for selectivelyexposing in fixed positions either said reflecting surface or saidnon-reflecting surface to electromagnetic energy received by saidreflecting system.

2. An aircraft energy reflecting system according to claim 1 whichfurther comprises a second reflecting unit, each of the reflecting unitsbeing located at the lateral extremities of the aircraft.

References Cited by the Examiner UNITED STATES PATENTS 2,310,790 2/1943Jungersen 343-18 2,770,801 11/1956 Jones 34318 2,917,739 12/1959 Halpern343-18 3,016,532 1/1962 Del Mar 343-18 3,039,093 6/1962 Rockwood 343-18CHESTER L. IUSTUS, Primary Examiner.

J. P. MORRIS, Assistant Examiner.

1. AN AIRCRAFT ELECTROMAGNETIC ENERGY REFLECTING SYSTEM COMPRISING: (A)FIRST REFLECTING UNIT INCLUDING A POLYHEDRON HAVING A REFLECTING SURFACEAND A NON-REFLECTING SURFACE, SAID REFLECTING AND NON-REFLECTINGSURFACES FACING IN OPPOSITE DIRECTIONS ALONG A COMMON AXIS OF SYMMETRYPASSING THROUGH SAID SURFACES, THE REFLECTING SURFACE HAVING AMULTIPLICITY OF SIDES SUFFICIENTLY NUMEROUS TO PRODUCE AN OVERALLREFLECTING CONFIGURATION THAT IS SUBSTANTIALLY HEMISPHERICAL, EACH OFSAID SIDES INCLUDING IN THE SURFACE THEREOF A DEPRESSED PORTION FORMINGA RE-ENTRANT ANGLE, ALL OF SAID DEPRESSED PORTIONS SERVING AS CORNERREFLECTORS; (B) AND MEANS INCLUDING A ROTATABLE AXLE AFFIXED TO SAIDPOLYHEDRON FOR ROTATING THE POLYHEDRON THROUGH AN ANGLE OF 180 DEGREESFOR SELECTIVELY EXPOSING IN FIXED POSITIONS EITHER SAID REFLECTINGSURFACE OR SAID NON-REFLECTING SURFACE TO ELECTROMAGNETIC ENERGYRECEIVED BY SAID REFLECTING SYSTEM.